Method for conditioning membrane-electrode-units for fuel cells

ABSTRACT

The present invention relates to a method for the conditioning of membrane electrode assemblies for fuel cells in which the output of the membrane electrode assemblies used can be increased and therefore the efficiency of the resulting polymer electrolyte membrane fuel cells can be improved.

RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 13/659,414, filed Oct. 24, 2012, which is incorporated byreference herein in its entirety, which was a continuation applicationof U.S. patent application Ser. No. 12/065,786, filed Mar. 5, 2008,which is a national stage application (under 35 U.S.C. 371) ofPCT/EP2006/008759 filed Sep. 8, 2006, which claims benefit of GermanApplication No. 10 2005 043 127.5 filed Sep. 10, 2005.

The present invention relates to a method for the conditioning ofmembrane electrode assemblies for fuel cells in which the output of themembrane electrode assemblies used can be increased and therefore theefficiency of the resulting polymer electrolyte membrane fuel cells canbe improved.

A fuel cell usually contains an electrolyte and two electrodes separatedby the electrolyte, in which one of the two electrodes is supplied witha fuel, such as hydrogen gas or a mixture of methanol and water, and theother electrode is supplied with an oxidant, such as oxygen gas or air.In the process, chemical energy arising from the resulting fueloxidation is directly converted into electric power.

One requirement of the electrolyte is that it is permeable to hydrogenions, i.e. protons, but not to the fuels mentioned above.

Typically, a fuel cell comprises several individual cells, so-calledMEAs (membrane electrode assembly), each of which contains anelectrolyte and two electrodes separated by the electrolyte. Aselectrolyte for the fuel cell, solids, such as polymer electrolytemembranes, or liquids, such as phosphoric acid, are applied. Polymerelectrolyte membranes have recently attracted interest as electrolytesfor fuel cells.

Polymer electrolyte membranes with complexes of alkaline polymers andstrong acids are known from WO96/13872, for example. To produce these,an alkaline polymer, for example polybenzimidazole, is treated with astrong acid, such as phosphoric acid.

Furthermore, fuel cells in which their membrane comprises inorganicsupport materials, such as for example glass-fibre fabrics orglass-fibre veils, which are saturated with phosphoric acid, are alsoknown (see U.S. Pat. No. 4,017,664).

In the alkaline polymer membranes known in the prior art, the mineralacid (mostly concentrated phosphoric acid) used—to achieve the requiredproton conductivity—is usually added following the forming of thepolyazole film. In doing so, the polymer serves as a support for theelectrolyte consisting of the highly concentrated phosphoric acid. Inthe process, the polymer membrane fulfils further essential functions,particularly, it has to exhibit a high mechanical stability and serve asa separator for the two fuels mentioned at the outset.

An essential advantage of such a membrane doped with phosphoric acid isthe fact that a fuel cell in which such a polymer electrolyte membraneis used can be operated at temperatures above 100° C. without thehumidification of the fuels otherwise necessary. This is due to thecharacteristic of the phosphoric acid to be able to transport theprotons without additional water via the so-called Grotthus mechanism(K.-D. Kreuer, Chem, Mater. 1996, 8, 610-641).

Further advantages for the fuel cell system are achieved through thepossibility of operation at temperatures above 100° C. On the one hand,the sensitivity of the Pt catalyst to gas impurities, in particular CO,is reduced substantially. CO is formed as a by-product in the reformingof hydrogen-rich gas from carbon-containing compounds, such as, e.g.,natural gas, methanol or benzine, or also as an intermediate product inthe direct oxidation of methanol. Typically, the CO content of the fuelhas to be lower than 100 ppm at temperatures <100° C. However, attemperatures in the range of 150-200°, 10,000 ppm CO or more can also betolerated (N. J. Bjerrum et. al., Journal of Applied Electrochemistry,2001, 31, 773-779). This results in substantial simplifications of theupstream reforming process and therefore reductions of the cost of theentire fuel cell system.

The output of a membrane electrode assembly produced with such membranesis described in WO 01/18894 and in Electrochimica Acta, Volume 41, 1996,193-197 and amounts to less than 0.2 A/cm² with a platinum loading of0.5 mg/cm² (anode) and 2 mg/cm² (cathode) and a voltage of 0.6 V. Whenusing air instead of oxygen, this value drops to less than 0.1 A/cm².

A big advantage of fuel cells is the fact that, in the electrochemicalreaction, the energy of the fuel is directly converted into electricpower and heat. In the process, water is formed at the cathode as areaction product. Heat is also produced in the electrochemical reactionas a by-product. In applications in which only the power for theoperation of electric motors is utilised, such as e.g. in automotiveapplications, or as a versatile replacement of battery systems, part ofthe heat generated in the reaction has to be dissipated to preventoverheating of the system. Additional energy-consuming devices whichfurther reduce the total electric efficiency of the fuel cell are thenneeded for cooling. In stationary applications, such as for thecentralised or decentralised generation of electricity and heat, theheat can be used efficiently by existing technologies, such as, e.g.,heat exchangers. In doing so, high temperatures are aimed for toincrease the efficiency. If the operating temperature is higher than100° C. and the temperature difference between the ambient temperatureand the operating temperature is high, it will be possible to cool thefuel cell system more efficiently, for example using smaller coolingsurfaces and dispensing with additional devices, in comparison to fuelcells which have to be operated at less than 100° C. due to thehumidification of the membrane.

The membrane electrode assemblies set forth above already show a goodproperty profile, however, the capability, for example the currentdensity at high voltages, of known membrane electrode assemblies has tobe improved further.

A further object was to provide a membrane electrode assembly whichexhibits a high capability, in particular a high current density or ahigh current density at a high voltage, respectively,

over a wide range of temperatures.

Additionally, the membrane electrode assembly according to the inventionhas to display high durability, in particular a long service life at thehigh power densities demanded.

An ongoing object in all membrane electrode assemblies is to lower thequantities of catalysts to minimise the production costs without therebyreducing the capability significantly. Advantageously, it should bepossible to operate the membrane electrode assembly with little gas flowand/or with low excess pressure achieving high power density.

Therefore, the present invention has the object to provide membraneelectrode assemblies which, on the one hand, meet the criteria set forthabove and, on the other hand, show an improved output.

The subject-matter of the present invention is a method for conditioninga membrane electrode assembly wherein a membrane electrode assemblycontaining

-   A) at least one polymer electrolyte matrix containing at least one    oxo acid of phosphorus and/or sulphur and at least one polymer with    at least one heteroatom selected from the group of nitrogen, oxygen    and/or sulphur,-   B) at least two electrodes,    is conditioned after the lamination of the polymer electrolyte    matrix and the electrodes in a temperature range of 60° C. to 300°    C.

A BRIEF DESCRIPTION OF THE FIGURE

FIG. 1 shows the current voltage characteristics of example 3.

The membrane electrode assembly resulting from the method according tothe invention shows an improved output in comparison with membraneelectrode assemblies, which have not been conditioned with the methodaccording to the invention.

The conditioning according to the invention is performed at temperaturesfrom 60° C. to 300° C., preferably from 80° C. to 300° C., in particularfrom 100° C. to 290° C., particularly preferably from 110° C. to 280°C., very particularly preferably from 140° C. to 275° C.

The minimum duration of the conditioning according to the invention isat least 30 seconds, preferably at least 1 minute, in particular atleast 2 minutes.

The duration of the conditioning is at most 24 hours. It is possible toalso operate the conditioning for longer than 24 hours; however, nosignificant improvement of the performance is observed in the process.

A treatment time of 30 seconds to 24 hours, preferably 1 minute to 20hours, in particular 2 minutes to 20 hours, is considered to be aneconomically sensible treatment time.

By means of the conditioning described above, the water content of thepolymer electrolyte matrix in the membrane electrode assembly of about26% by weight is significantly reduced. It has been shown that theimprovement of the performance starts when the water content isdecreased to 20% by weight and less. After conditioning, the watercontent of the polymer electrolyte matrix in the membrane electrodeassembly is therefore less than 20% by weight.

Inasmuch as the conditioning according to the invention of the membraneelectrode assembly is performed in the installed cell, it isadvantageous to flush the cell and therefore also the membrane electrodeassembly with at least one gaseous medium, in this manner, waterpossibly present is discharged and the residual water content describedabove is obtained.

In a conditioning in the cell, this is substantially performed undercurrentless conditions, i.e. at most a current which corresponds to 10mA/cm² at 800 mV is drawn during the conditioning. Thus, theconditioning can also be performed within the scope of a controlledstart of the cell.

As gaseous media, all gaseous media, preferably air, oxygen, nitrogenand/or noble gases, such as argon, helium, are suitable. Preferably,gaseous media which contain no hydrogen gas or develop no hydrogen gasunder the chosen conditions and therefore undergo electrochemicalreactions are used.

Inasmuch as the conditioning according to the invention of the membraneelectrode assembly is performed in a state in which it is not installed,the flushing process with a gaseous medium can also be dispensed with.

The membrane electrode assemblies conditioned according to the inventioncontain at least one polymer electrolyte matrix, which in turn containsat least one polymer with at least one heteroatom selected from thegroup of nitrogen, oxygen and/or sulphur. The polymers are preferablyalkaline polymers.

The alkaline polymers are preferably polymers, which comprise at leastone nitrogen atom.

The alkalinity of the polymer can also be defined via the molar ratio ofnitrogen atoms to carbon atoms. The scope of the present inventionencompasses in particular such polymers whose molar ratio of nitrogenatoms to carbon atoms is in the range of 1:1 to 1:100, preferably in therange of 1:2 to 1:20, This ratio can be determined by elementalanalysis.

Alkaline polymers, in particular polymers with at least one nitrogenatom, are known in professional circles. In general, polymers with onenitrogen atom in the backbone and/or in the side chain can be used.

The polymers with one nitrogen atom include, for example,polyphosphazenes, polyimines, polyisocyanides, polyetherimine,polyaniline, polyamides, polyhydrazides, polyurethanes, polyimides,polyazoles and/or polyazines.

Preferably, the polymer membranes comprise polymers with at least onenitrogen atom used in a repeating unit. In this connection, it is alsopossible to use copolymers which, in addition to repeating units withone nitrogen atom, also comprise repeating units without a nitrogenatom.

According to a particular aspect of the present invention, alkalinepolymers with at least one nitrogen atom are used. The term alkaline isknown in professional circles in which this is to be understood inparticular as Lewis and Brønstedt alkalinity.

The repeating unit in the alkaline polymer preferably contains anaromatic ring with at least one nitrogen atom. The aromatic ring ispreferably a five- to six-membered ring with one to three nitrogen atomswhich can be fused to another ring, in particular another aromatic ring.

Polymers based on polyazole generally contain recurring azole units ofthe general formula (I) and/or (II) and/or (Ill) and/or (IV) and/or (V)and/or (VI) and/or (VII) and/or (VIII) and/or (IX) and/or (X) and/or(XI) and/or (XII) and/or (XIII) and/or (XIV) and/or (XV) and/or (XVI)and/or (XVII) and/or (XVIII) and/or (XIX) and/or (XX) and/or (XXI)and/or (XXII).

wherein

-   Ar are the same or different and are each a tetravalent aromatic or    heteroaromatic group which may be mononuclear or polynuclear,-   Ar¹ are the same or different and are each a divalent aromatic or    heteroaromatic group which may be mononuclear or polynuclear,-   Ar² are the same or different and are each a divalent or trivalent    aromatic or heteroaromatic group which may be mononuclear or    polynuclear,-   Ar³ are the same or different and are each a trivalent aromatic or    heteroaromatic group which may be mononuclear or polynuclear,-   Ar⁴ are the same or different and are each a trivalent aromatic or    heteroaromatic group which may be mononuclear or polynuclear,-   Ar⁵ are the same or different and are each a tetravalent aromatic or    heteroaromatic group which may be mononuclear or polynuclear,-   Ar⁶ are the same or different and are each a divalent aromatic or    heteroaromatic group which may be mononuclear or polynuclear,-   Ar⁷ are the same or different and are each a divalent aromatic or    heteroaromatic group which may be mononuclear or polynuclear,-   Ar⁸ are the same or different and are each a trivalent aromatic or    heteroaromatic group which may be mononuclear or polynuclear,-   Ar⁹ are the same or different and are each a divalent or trivalent    or tetravalent aromatic or heteroaromatic group which may be    mononuclear or polynuclear,-   Ar¹⁰ are the same or different and are each a divalent or trivalent    aromatic or heteroaromatic group which may be mononuclear or    polynuclear,-   Ar¹¹ are the same or different and are each a divalent aromatic or    heteroaromatic group which may be mononuclear or polynuclear,-   X are the same or different and are each oxygen, sulphur or an amino    group which bears a hydrogen atom, a group having 1-20 carbon atoms,    preferably a branched or unbranched alkyl or alkoxy group, or an    aryl group as further radical,-   R are identical or different and represent hydrogen, an alkyl group    and an aromatic group, with the proviso that R in the formula (XX)    is not hydrogen, and-   n, m are each an integer greater than or equal to 10, preferably    greater or equal to 100.

Aromatic or heteroaromatic groups which are preferred according to theinvention are derived from benzene, naphthalene, biphenyl, diphenylether, diphenylmethane, diphenyldimethylmethane, bisphenone,diphenylsulphone, thiophene, furan, pyrrole, thiazole, oxazole,imidazole, isothiazole, isoxazole, pyrazole, 1,3,4-oxadiazole,2,5-diphenyl-1,3,4-oxadiazole, 1,3,4-thiadiazole, 1,3,4-triazole,2,5-diphenyl-1,3,4-triazole, 1,2,5-triphenyl-1,3,4-triazole,1,2,4-oxadiazole, 1,2,4-thiadiazole, 1,2,4-triazole, 1,2,3-triazole,1,2,3,4-tetrazole, benzo[b]thiophene, benzo[b]furan, indole,benzo[c]thiophene, benzo[c]furan, isoindole, benzoxazole, benzothiazole,benzimidazole, benzisoxazole, benzisothiazole, benzopyrazole,benzothiadiazole, benzotriazole, dibenzofuran, dibenzothiophene,carbazole, pyridine, bipyridine, pyrazine, pyrazole, pyrimidine,pyridazine, 1,3,5-triazine, 1,2,4-triazine, 1,2,4,5-triazine, tetrazine,quinoline, isoquinoline, quinoxaline, quinazoline, cinnoline,1,8-naphthyridine, 1,5-naphthyridine, 1,6-naphthyridine,1,7-naphthyridine, phthalazine, pyridopyrimidine, purine, pteridine orquinolizine, 4H-quinolizine, diphenyl ether, anthracene, benzopyrrole,benzooxathiadiazole, benzooxadiazole, benzopyridine, benzopyrazine,benzopyrazidine, benzopyrimidine, benzotriazine, indolizine,pyridopyridine, imidazopyrimidine, pyrazinopyrimidino, carbazole,acridine, phenazine, benzoquinoline, phenoxazine, phenothiazine,acridizine, benzopteridine, phenanthroline and phenanthrene, each ofwhich may optionally also be substituted.

In this case, Ar¹, Ar⁴, Ar⁶, Ar⁷, Ar⁸, Ar⁹, Ar¹⁰, Ar¹¹ can have anysubstitution pattern, in the case of phenylene, for example, Ar¹, Ar⁴,Ar⁶, Ar⁷, Ar⁸, Ar⁹, Ar¹⁰, Ar¹¹ can be ortho-, meta- and para-phenylene.Particularly preferred groups are derived from benzene and biphenylene,which may also be substituted.

Preferred alkyl groups are short-chain alkyl groups having 1 to 4 carbonatoms, e.g. methyl, ethyl, n- or i-propyl and t-butyl groups.

Preferred aromatic groups are phenyl or naphthyl groups. The alkylgroups and the aromatic groups can be substituted.

Preferred substituents are halogen atoms such as, e.g., fluorine, aminogroups, hydroxyl groups or short-chain alkyl groups such as, e.g.,methyl or ethyl groups.

Polyazoles having recurring units of the formula (I) are preferredwherein the radicals X within one recurring unit are identical.

The polyazoles can in principle also have different recurring unitswherein their radicals X are different, for example. It is preferable,however, that a recurring unit has only identical radicals X.

Further preferred polyazole polymers are polyimidazoles,polybenzothiazoles, polybenzoxazoles, polyoxadiazoles, polyquinoxalines,polythiadiazoles, poly(pyridines), poly(pyrimidines) andpoly(tetrazapyrenes).

In another embodiment of the present invention, the polymer containingrecurring azole units is a copolymer or a blend which contains at leasttwo units of the formulae (I) to (XXII) which differ from one another.The polymers can be in the form of block copolymers (diblock, triblock),random copolymers, periodic copolymers and/or alternating polymers.

In a particularly preferred embodiment of the present invention, thepolymer containing recurring azole units is a polyazole which onlycontains units of the formulae (I) and/or (II).

The number of recurring azole units in the polymer is preferably aninteger greater than or equal to 10. Particularly preferred polymerscontain at least 100 recurring azole units.

Within the scope of the present invention, polymers containing recurringbenzimidazole units are preferred. Some examples of the most purposefulpolymers containing recurring benzimidazole units are represented by thefollowing formulae:

where n and m are each an integer greater than or equal to 10,preferably greater than or equal to 100.

Further preferred polyazole polymers are polyimidazoles,polybenzirnidazole ether ketone, polybenzothiazoles, polybenzoxazoles,polytriazoles, polyoxadiazoles, polythiadiazoles, polypyrazoles,polyquinoxalines, poly(pyridines), poly(pyrimidines) andpoly(tetrazapyrenes).

Preferred polyazoles are characterized by a high molecular weight. Thisapplies in particular to the polybenzimidazoles. Measured as theintrinsic viscosity, this is in the range of 0.3 to 10 dl/g, preferably1 to 5 dl/g.

Celazole from the company Celanese is particularly preferred. Theproperties of the polymer film and polymer membrane can be improved byscreening the starting polymer, as described in German patentapplication No. 10129458 A1.

Very particular preference is given to using para-polybenzimidazoles inthe production of the polymer electrolyte membranes. In this connection,the polybenzimidazoles comprise in particular six-membered aromaticgroups which are linked at the 1,4 position. Particular preference isgiven to using poly-[2,2′-(p-phenylene)-5,5′-bisbenzimidazole].

The polymer film used for the doping and based on alkaline polymers cancomprise still more additives in the form of fillers and/or auxiliaries.Additionally, the polymer film can feature further modifications, forexample by cross-linking, as described in the German patent applicationDE 10110752 A1 or in WO 00/44816. In a preferred embodiment, the polymerfilm used for the doping and consisting of an alkaline polymer and atleast one blend component additionally contains a cross-linking agent,as described in the German patent application DE 10140147 A1. Anessential advantage of such a system is the fact that higher dopinglevels and therefore a greater conductivity with sufficient mechanicalstability of the membrane can be achieved.

In addition to the above-mentioned alkaline polymers, a blend of one ormore alkaline polymers with another polymer can be used. In this case,the function of the further blend component is essentially to improvethe mechanical properties and reduce the cost of material.

Preferred blend components include polysulphones, in particularpolysulphone having aromatic and/or heteroaromatic groups in thebackbone. According to a particular aspect of the present invention,preferred polysulphones and polyethersulphones have a melt volume rateMVR 300/21.6 of less than or equal to 40 cm³/10 min, in particular lessthan or equal to 30 cm³/10 min and particularly preferably less than orequal to 20 cm³/10 min, measured in accordance with ISO 1133. In thisconnection, polysulphones with a Vicat softening point VST/A/50 of 180°C. to 230° C. are preferred. In yet another preferred embodiment of thepresent invention, the number average of the molecular weight of thepolysulphones is greater than 30,000 g/mol.

The polymers based on polysulphone include in particular polymers havingrecurring units with linking sulphone groups according to the generalformulae A, B, C, D, E, F and/or G:

wherein the radicals R, independently of another, identical ordifferent, represent aromatic or heteroaromatic groups, these radicalshaving been explained in detail above. These include in particular1,2-phenylene, 1,3-phenylene, 1,4-phenylene, 4,4′-biphenyl, pyridine,quinoline, naphthalene, phenanthrene.

The polysulphones preferred within the scope of the present inventioninclude homopolymers and copolymers, for example random copolymers.Particularly preferred polysulphones comprise recurring units of theformulae H to N:

where n>o

where n<o

The previously described polysulphones can be obtained commerciallyunder the trade names®Victrex 200 P,®Victrex 720 P,®Ultrason E,®UltrasonS,®Mindel,®Radel A,®Radel R,®Victrex HTA,®Astrel and®Udel.

Furthermore, polyether ketones, polyether ketone ketones, polyetherether ketones, polyether ether ketone ketones and polyaryl ketones areparticularly preferred. These high-performance polymers are known per seand can be obtained commercially under the trade names Victrex®PEEK™,®Hostatec,®Kadel.

Furthermore, polymeric blend components which contain acid groups can beused. These acid groups comprise in particular sulphonic acid groups.Here, polymers containing aromatic sulphonic acid groups can be usedwith preference.

Aromatic sulphonic acid groups are groups in which the sulphonic acidgroup (—SO₃H) is covalently bonded to an aromatic or heteroaromaticgroup. The aromatic group can be part of the backbone of the polymer orpart of a side group wherein polymers having aromatic groups in thebackbone are preferred. In many cases, the sulphonic acid groups canalso be used in the form of their salts. Furthermore, derivatives, forexample esters, in particular methyl or ethyl esters, or halides of thesulphonic acids can be used which are converted to the sulphonic acidduring operation of the membrane.

The polymers modified with sulphonic acid groups preferably have acontent of sulphonic acid groups in the range from 0.5 to 3 meq/g. Thisvalue is determined by way of the so-called ion exchange capacity (IEC).

To measure the IEC, the sulphonic acid groups are converted to the freeacid. To this end, the polymer is treated in a known way with acid,removing excess acid by washing. Thus, the sulphonated polymer isinitially treated for 2 hours in boiling water. Subsequently, excesswater is dabbed off and the sample is dried at 160° C. in a vacuumdrying cabinet at p<1 mbar for 15 hours. Then, the dry weight of themembrane is determined. The polymer thus dried is then dissolved in DMSOat 80° C. for 1 h. Subsequently, the solution is titrated with 0.1 MNaOH. The ion exchange capacity (IEC) is then calculated from theconsumption of acid up to the equivalent point and the dry weight.

Such polymers are known by those in the field. Polymers containingsulphonic acid groups can be produced, for example, by sulphonation ofpolymers. Methods for the sulphonation of polymers are described in F.Kucera et al., Polymer Engineering and Science 1988, Vol. 38, No, 5,783-792. In this connection, the sulphonation conditions can be chosensuch that a low degree of sulphonation develops (DE-A-19959289).

A further class of non-fluorinated polymers has been developed bysulphonation of high-temperature-stable thermoplasts. For example,sulphonated polyether ketones (DE-A-4219077, WO96/01177), sulphonatedpolysulphones (J. Membr. Sci. 83 (1993) p. 211) or sulphonatedpolyphenylene sulphide (DE-A-19527435) are known.

U.S. Pat. No. 6,110,616 describes copolymers of butadiene and styreneand the subsequent sulphonation thereof for use for fuel cells.

Furthermore, such polymers can also be obtained by polyreactions ofmonomers, which comprise acid groups. Thus, perfluorinated polymers asdescribed in U.S. Pat. No. 5,422,411 can be produced by copolymerisationof trifluorostyrene and sulphonyl-modified trifluorostyrene.

These perfluorosulphonic acid polymers include inter alia Nafion® (U.S.Pat. No. 3,692,569). This polymer can—as described in U.S. Pat. No.4,453,991—be brought into solution and then used as an ionomer.

The preferred polymers containing acid groups include inter aliasulphonated polyether ketones, sulphonated polysulphones, sulphonatedpolyphenylene sulphides, perfluorinated sulphonic acid group-containingpolymers, as described in U.S. Pat. No. 3,692,569, U.S. Pat. No.5,422,411 and U.S. Pat. No. 6,110,616.

Besides the above-mentioned blend, polyolefines, such aspoly(chloroprene), polyacetylene, polyphenylene, poly(p-xylylene),polyarylmethylene, polyarmethylene, polystyrene, polymethylstyrene,polyvinyl alcohol, polyvinyl acetate, polyvinyl ether, polyvinyl amine,poly(N-vinyl acetamide), polyvinyl imidazole, polyvinyl carbazole,polyvinyl pyrrolidone, polyvinyl pyridine, polyvinyl chloride,polyvinylidene chloride, polytetrafluoroethylene,polyhexafluoropropylene, copolymers of PTFE with hexafluoropropylene,with perfluoropropylvinyl ether, with trifluoronitrosomethane, withsulphonyl fluoride vinyl ether, with carbalkoxyperfluoroalkoxyvinylether, polychlorotrifluoroethylene, polyvinyl fluoride, polyvinylidenefluoride, polyacrolein, polyacrylamide, polyacrylonitrile,polycyanoacrylates, polymethacrylimide, cycloolefinic copolymers, inparticular of norbornenes; polymers having C—O bonds in the backbone,for example polyacetal, polyoxymethylene, polyether, polypropyleneoxide, polyepichlorohydrin, polytetrahydrofuran, polyphenylene oxide,polyether ketone, polyester, in particular polyhydroxyacetic acid,polyethyleneterephthalate, polybutyleneterephthalate,polyhydroxybenzoate, polyhydroxypropionic acid, polypivalolacton,polycaprolacton, polymalonic acid, polycarbonate; polymeric C—S bonds inthe backbone, for example polysulphide ether, polyphenylenesulphide,polyethersulphone; polymers containing C—N bonds in the backbone, forexample polyimines, polyisocyanides, polyetherimine, polyaniline,polyamides, polyhydrazides, polyurethanes, polyimides, polyazoles,polyazines; liquid crystalline polymers, in particular Vectra, as wellas inorganic polymers, such as polysilanes, polycarbosilanes,polysiloxanes, polysilicic acid, polysilicates, silicons,polyphosphazenes and polythiazyl, can also be used.

By using blends, the mechanical properties can be improved and thematerial costs can be reduced.

Additionally—as already mentioned above the blend polymer film can alsofeature further modifications, for example by cross-linking, asdescribed in the German patent application DE 10110752 A1 or in WO00/44816. In a preferred embodiment, the polymer film used for theswelling and consisting of an alkaline polymer and at least one blendcomponent additionally contains a cross-linking agent, as described inthe German patent application DE 10140147 A1 .

In order to produce polymer films, the aforementioned polymers may interalia be extruded. Polymer films can also be obtained by means of castingmethods. For example, polyazoles can be dissolved in polar, aproticsolvents, such as dimethylacetamide (DMAc) for example, and a film canbe produced by conventional methods.

To remove solvent residues, such as DMAc, the film thus obtained can becleaned by means of a washing process.

Polymer electrolyte matrices based on alkaline polymers, as described inDE 10117686 A1, DE 10117687 A1 and DE 10144815 A1, DE 10228657 A1 DE10246373 A1 and DE 10246459 A1, have been shown to be particularly wellsuited.

Besides the above-described polymer electrolyte matrices made ofalkaline polymers and blends made of alkaline polymers and furtherpolymers, other materials can also be used.

Here, in particular polymer electrolyte matrices based on alkalinepolymers and polymers based on vinylsulphone/vinylphosphone, asdescribed in DE 10213540 A1, DE 10209419 A1 and DE 10210500 A1, DE10210499 A1, DE 10235358 A1, DE 10235357 A1 and DE 10235356 A1, areshown.

With the above-mentioned polymer electrolyte matrices based on alkalinepolymers and polymers based on vinylsulphone/vinylphosphone, thealkaline polymers mentioned above in connection with polymers based onvinylsulphone/vinylphosphone are produced. Polymers based onvinylsulphone/vinylphosphone are understood to mean polymers, which areobtained using monomers comprising phosphonic acid groups of the formula

wherein

-   R represents a bond, a bicovalent C1-C15 alkylene group, a    bicovalent C1-C15 alkyleneoxy group, for example ethyleneoxy group,    or a bicovalent C5-C20 aryl or heteroaryl group wherein the    above-mentioned radicals themselves can be substituted with halogen,    —OH, COOZ, —CN, NZ₂,-   Z represent, independently of another, hydrogen, a C1-C15 alkyl    group, a C1-C15 alkoxy group, an ethyleneoxy group or a C5-C20 aryl    or heteroaryl group wherein the above-mentioned radicals themselves    can be substituted with halogen, —OH, —ON, and-   x represents an integer 1, 2, 3, 4, 5, 6, 7, 8, 9 or 10-   y represents an integer 1, 2, 3, 4, 5, 6, 7, 8, 9 or 10    and/or of the formula

wherein

-   represents a bond, a bicovalent C1-C15 alkylene group, a bicovalent    C1-C15 alkyleneoxy group, for example ethyleneoxy group, or a    bicovalent C5-C20 aryl or heteroaryl group wherein the    above-mentioned radicals themselves can be substituted with halogen,    —OH, COOZ, —CN, NZ₂,-   Z represent, independently of another, hydrogen, a C1-C15 alkyl    group, a C1-C15 alkoxy group, an ethyleneoxy group or a C5-C20 aryl    or heteroaryl group wherein the above-mentioned radicals themselves    can be substituted with halogen, —OH, —CN, and-   x represents an integer 1, 2, 3, 4, 5, 6, 7, 8, 9or 10    and/or of the formula

wherein

-   A represents a group of the formulae COOR², CN, CONR² ₂, OR² and/or    R², in which R² is hydrogen, a C1-C15 alkyl group, a C1-C15 alkoxy    group, an ethylenoxy group or a C5-C20 aryl or heteroaryl group,    wherein the above radicals may in turn be substituted by halogen,    —OH, COOZ, —CN, NZ₂,-   R represents a bond, a bicovalent C1-C15 alkylene group, a    bicovalent C1-C15 alkyleneoxy group, for example ethyleneoxy group,    or a bicovalent C5-C20 aryl or heteroaryl group wherein the    above-mentioned radicals themselves can be substituted with halogen,    —OH, COOZ, —CN, NZ₂,-   Z represent, independently of another, hydrogen, a C1-C15 alkyl    group, a C1-C15 alkoxy group, an ethyleneoxy group or a C5-C20 aryl    or heteroaryl group wherein the above-mentioned radicals themselves    can be substituted with halogen, —OH, —CN, and-   x represents an integer 1, 2, 3, 4, 5, 6, 7, 8, 9 or 10,    and/or using monomers comprising sulphonic acid groups of the    formula

wherein

-   R represents a bond, a bicovalent C1-C15 alkylene group, a    bicovalent C1-C15 alkyleneoxy group, for example ethyleneoxy group,    or a bicovalent C5-C20 aryl or heteroaryl group wherein the    above-mentioned radicals themselves can be substituted with halogen,    —OH, COOZ, —CN, NZ₂,-   Z represent, independently of another, hydrogen, a C1-C15 alkyl    group, a C1-C15 alkoxy group, an ethyleneoxy group or a C5-C20 aryl    or heteroaryl group wherein the above-mentioned radicals themselves    can be substituted with halogen, —OH, —CN, and-   x represents an integer 1, 2, 3, 4, 5, 6, 7, 8, 9 or 10-   y represents an integer 1, 2, 3, 4, 5, 6, 7, 8, 9 or 10    and/or of the formula

wherein

-   R represents a bond, a bicovalent C1-C15 alkylene group, a    bicovalent C1-C15 alkyleneoxy group, for example ethyleneoxy group,    or a bicovalent C5-C20 aryl or heteroaryl group wherein the    above-mentioned radicals themselves can be substituted with halogen,    —OH, COOZ, —CN, NZ₂,-   Z represent, independently of another, hydrogen, a C1-C15 alkyl    group, a C1-C15 alkoxy group, an ethyleneoxy group or a C5-C20 aryl    or heteroaryl group wherein the above-mentioned radicals themselves    can be substituted with halogen, —OH, —CN, and-   x represents an integer 1, 2, 3, 4, 5, 6, 7, 8, 9 or 10    and/or of the formula

wherein

-   A represents a group of the formulae COOR², CN, CONR² ₂, OR² and/or    R², in which R² is hydrogen, a C1-C15 alkyl group, a C1-C15 alkoxy    group, an ethylenoxy group or a C5-C20 aryl or heteroaryl group,    wherein the above radicals may in turn be substituted by halogen,    —OH, COOZ, —CN, NZ₂,-   R represents a bond, a bicovalent C1-C15 alkylene group, a    bicovalent C1-C15 alkyleneoxy group, for example ethyleneoxy group,    or a bicovalent C5-C20 aryl or heteroaryl group wherein the    above-mentioned radicals themselves can be substituted with halogen,    —OH, COOZ, —ON, NZ₂,-   Z represent, independently of another, hydrogen, a C1-C15 alkyl    group, a C1-C15 alkoxy group, an ethyleneoxy group or a C5-C20 aryl    or heteroaryl group wherein the above-mentioned radicals themselves    can be substituted with halogen, —OH, —CN, and-   x represents an integer 1, 2, 3, 4, 5, 6, 7, 8, 9 or 10,    as described in DE 10213540 A1, DE 10209419 A1 and DE 10210500 A1,    DE 10210499 A1, DE 10235358 A1, DE 10235357 A1 and DE 10235356 A1,    for example.

Preferred monomers comprising phosphonic acid groups include, amongstothers, alkenes having phosphonic acid groups, such as ethenephosphonicacid, propenephosphonic acid, butenephosphonic acid; acrylic acid and/ormethacrylic acid compounds having phosphonic acid groups, such as forexample 2-phosphonomethyl acrylic acid, 2-phosphonomethyl methacrylicacid, 2-phosphonomethyl acrylamide and 2-phosphonomethyl methacrylamide.

Commercially available vinylphosphonic acid (ethenephosphonic acid),such as it is available from the company Aldrich or Clariant GmbH, forexample, is particularly preferably used. A preferred vinyiphosphonicacid has a purity of more than 70%, in particular 90% and particularlypreferably a purity of more than 97%.

The monomers containing phosphonic acid groups may also be used in theform of derivatives which can subsequently be converted into the acid,wherein the conversion to acid may also take place in the polymerisedstate. These derivatives include in particular the salts, esters, amidesand halides of the monomers containing phosphonic acid groups.

Preferred monomers comprising sulphonic acid groups include, amongstothers, alkenes having sulphonic acid groups, such as ethenesulphonicacid, propenesulphonic acid, butenesulphonic acid; acrylic acidcompounds and/or methacrylic acid compounds having sulphonic acidgroups, such as for example 2-sulphonomethyl acrylic acid,2-sulphonomethyl methacrylic acid, 2-sulphonomethyl acrylamide and2-sulphonomethyl methacrylamide.

Commercially available vinylsulphonic acid (ethenesulphonic acid), suchas it is available from the company Aldrich or Clariant GmbH, forexample, is particularly preferably used. A preferred vinylsulphonicacid has a purity of more than 70%, in particular 90% and particularlypreferably a purity of more than 97%.

The monomers containing sulphonic acid groups can furthermore be used inthe form of derivatives which can subsequently be converted to the acid,wherein the conversion to acid may also take place in the polymerisedstate. These derivatives include in particular the salts, esters, amidesand halides of the monomers comprising sulphonic acid groups.

According to a particular aspect of the present invention, the weightratio of monomers comprising sulphonic acid groups to monomerscomprising phosphonic acid groups can be in the range of 100:1 to 1:100,preferably 10:1 to 1:10 and particularly preferably 2:1 to 1:2.

In another embodiment of the invention, monomers capable ofcross-linking can be used in the production of the polymer membrane.

The monomers capable of cross-linking are in particular compounds havingat least 2 carbon-carbon double bonds. Preference is given to dienes,trienes, tetraenes, dimethylacrylates, trimethylacrylates,tetramethylacrylates, diacrylates, triacrylates, tetraacrylates.

Particular preference is given to dienes, trienes, tetraenes of theformula

dimethylacrylates, trimethylacrylates, tetramethylacrylates of theformula

diacrylates, triacrylates, tetraacrylates of the formula

wherein

-   R represents a C1-C15 alkyl group, a C5-C20 aryl or heteroaryl    group, NR′, —SO₂, PR′, Si(R′)₂, wherein the above-mentioned radicals    themselves can be substituted,-   R represent, independently of another, hydrogen, a C1-C15 alkyl    group, a C1-C15 alkoxy group, a C5-C20 aryl or heteroaryl group, and-   n is at least 2.    The substituents of the above radical R are preferably halogen,    hydroxyl, carboxy, carboxyl, carboxyl ester, nitrile, amine, silyl    or siloxane radicals.

Particularly preferred cross-linking agents are allyl methacrylate,ethylene glycol dimethacrylate, diethylene glycol dimethacrylate,triethylene glycol dimethacrylate, tetraethylene glycol dimethacrylateand polyethylene glycol dimethacrylate, 1,3-butanediol dimethacrylate,glycerol dimethacrylate, diurethane dimethacrylate, trimethylpropanetrimethacrylate, epoxy acrylates, for example ebacryl,N′,N-methylenebisacrylamide, carbinol, butadiene, isoprene, chloroprene,divinylbenzene and/or bisphenol A dimethylacryiate. These compounds arecommercially available from Sartomer Company Exton, Pa. under thedesignations ON-120, CN104 and CN-980, for example.

The use of cross-linking agents is optional, wherein these compounds cantypically be employed in the range of 0.05 and 30% by weight, preferably0.1 to 20% by weight, particularly preferably 1 to 10% by weight, basedon the weight of the monomers comprising phosphonic acid groups.

A fundamental aspect and technical advantage of the membrane electrodeassembly according to the invention is that the excellent capability isobtained with a low concentration of catalytically active substances,such as for example platinum, ruthenium or palladium, heretofore notachieved.

To further improve the properties in terms of application technology,the flat material can feature fillers, in particular proton-conductingfillers.

Non-limiting examples of proton-conducting fillers are

-   sulphates, such as CsHSO₄, Fe(SO₄)₂, (NH₄)₃H(SO₄)₂, LiHSO₄, NaHSO₄,    KHSO₄, RbSO₄, LiN₂H₅SO₄, NH₄HSO₄,-   phosphates, such as Zr₃(PO₄)4, Zr(HPO₄)₂, HZr₂(PO₄)₃, UO₂PO₄.3H₂O,    H₈UO₂PO₄, Ce(HPO₄)₂, Ti(HPO₄)₂, KH₂PO₄, NaH₂PO₄, LiH₂PO₄, NH₄H₂PO₄,    CsH₂PO₄, CaHPO₄, MgHPO₄, HSbP₂O₈, HSb₃P₂O₁₄, H₅Sb₅P₂O₂₀,-   polyacid, such as H₃PW₁₂O₄₀.nH₂O (n=21-29), H₃SiW₁₂O₄₀.nH₂O    (n=21-29), H_(x)WO₃, HSbWO₆, H₃PMo₁₂O₄₀, H₂Sb₄O₁₁, HTaWO₆, HNbO₃,    HTiNbO₅, HTiTaO₅, HSbTeO₆, H₅Ti₄O₉, HSbO₃, H₂MoO₄-   selenides and arsenides such as (NH₄)₃H(SeO₄)₂, UO₂AsO₄,    (NH₄)₃H(SeO₄)₂, KH₂AsO₄, Cs₃H(SeO₄)₂, Rb₃H(SeO₄)₂,-   phosphides, such as ZrP, TiP, HfP-   oxides, such as Al₂O₃, Sb₂O₅, ThO₂, SnO₂, ZrO₂, MoO₃-   silicates, such as zeolites, zeolites(NH₄+), phyllosilicates,    tectosilicates, H-natrolites, H-mordenites, NH₄-analcines,    NH₄-sodalites, NH₄-gallates, H-montmorillonites-   acids, such as HClO₄, SbF₅-   fillers, such as carbides, in particular SiC, Si₃N₄, fibres, in    particular glass fibres, glass powders and/or polymer fibres,    preferably based on polyazoles.

These additives can be included in the proton-conducting polymermembrane in usual amounts, however, the positive properties of themembrane, such as great conductivity, long service life and highmechanical stability, should not be affected too much by the addition oftoo large amounts of additives. Generally, the membrane comprises notmore than 80% by weight, preferably not more than 50% by weight andparticularly preferably not more than 20% by weight, of additives.

The membrane electrode assemblies conditioned according to the inventioncontain at least one oxo acid of phosphorus and/or sulphur in thepolymer electrolyte matrix. The acids identified above are strong acids,in particular mineral acids, particularly preferably phosphoric acidand/or sulphuric acid as well as their derivatives.

Within the scope of the present description, “phosphoric acid” meanspolyphosphoric acid (H_(n+2)P_(n)O_(3n+1)(n>1), which usually has acontent of at least 83%, calculated as P₂O₅ (by acidimetry), phosphonicacid (H₃PO₃), orthophosphoric acid (H₃PO₄), pyrophosphoric acid(H₄P₂O₇), triphosphoric acid (H₅P₃O₁₀) and metaphosphoric acid. Thephosphoric acid, in particular orthophosphoric acid, preferably has aconcentration of at least 80 percent by weight. Furthermore, the termphosphoric acid comprises also such compounds that release correspondingphosphoric acids during use in the fuel cell or form these by means ofbuild-up or degradation. This is in particular to be understood to meanorganic phosphoric acids and their derivatives, respectively.

The polymer electrolyte matrix used according to the invention isproton-conducting. Furthermore, the polymer electrolyte matrix has atleast 6 mole of acid per polymer repeating unit, preferably at least 8mole, particularly preferably at least 14 mole.

The thickness of the polymer electrolyte matrix in the membraneelectrode assembly preferably between 5 and 4000 μm, preferably between10 and 3500 μm, in particular between 20 and 3000 μm, particularlypreferably between 30 and 1500 μm and very particularly preferablybetween 50 and 1200 μm.

The polymer electrolyte matrix shows high proton conductivity. This isat least 0.1 S/cm, preferably at least 0.11 S/cm, in particular at least0.12 S/cm at temperatures of 120° C.

The specific conductivity is measured by means of impedancy spectroscopyin a 4-pole arrangement in potentiostatic mode and using platinumelectrodes (wire, diameter of 0.25 mm), as described in DE 10117687 A1.

A membrane electrode assembly according to the invention comprises, inaddition to the polymer membrane, at least two electrodes which are eachin contact with the membrane.

Generally, the electrode comprises a gas diffusion layer. The gasdiffusion layer in general exhibits electron conductivity. Flat,electrically conductive and acid-resistant structures are commonly usedfor this. These include, for example, carbon-fibre paper, graphitisedcarbon-fibre paper, carbon-fibre fabric, graphitised carbon-fibre fabricand/or flat structures which were rendered conductive by addition ofcarbon black.

Furthermore, the electrode includes at least one catalyst layer. Thislayer contains at least one precious metal of the platinum group, inparticular Pt, Pd, in Rh, Os, Ru, and/or at least one precious metal Auand/or Ag, or the catalyst layer is formed from

-   i. at least one precious metal of the platinum group, in particular    Pt, Pd, Ir, Rh, Os, Ru, and/or at least one precious metal Au and/or    Ag-   ii. and at least one metal less precious according to the    electrochemical series as the metal mentioned in (i.), in particular    selected from the group of Fe, Co, Ni, Cr, Mn, Zr, Ti, Ga, V.

Preferably, the catalyst is formed in the form of an alloy of the metals(i) and (ii). In addition to the alloy, further catalytically activesubstances, in particular precious metals of the platinum group, i.e.Pt, Pd, Ir, Rh, Os, Ru, or also the precious metals Au and Ag, can beused. Furthermore, the oxides of the above-mentioned precious metalsand/or non-precious metals can also be used.

The catalytically active particles comprising the above-mentionedsubstances may be used as metal powder, so-called black precious metal,in particular platinum and/or platinum alloys. Such particles generallyhave a size in the range of 5 nm to 200 nm, preferably in the range of 7nm to 100 nm.

Furthermore, the metals can also be used on a support material.Preferably, this support comprises carbon which particularly can be usedin the form of carbon black, graphite or graphitised carbon black.Furthermore, electrically conductive metal oxides, such as for example,SnO_(x), TiO_(x), or phosphates, such as e.g. FePO_(x), NbPO_(x),Zr_(y)(PO_(x))_(z), can be used as support material. In this connection,the indices x, y and z designate the oxygen or metal content of theindividual compounds which can lie within a known range as thetransition metals can be in different oxidation stages.

The content of these metal particles on a support, based on the totalweight of the bond of metal and support, is generally in the range of 1to 80% by weight. The particle size of the support, in particular thesize of the carbon particles, is preferably in the range of 20 to 100nm. The size of the metal particles present thereon is preferably in therange of 1 to 20 nm.

The catalyst layer has a thickness in the range of 0.1 to 50 μm.

The membrane electrode assembly according to the invention has acatalyst loading of 0.1 and 10 g/m², based on the surface of the polymerelectrolyte matrix.

The weight ratio of the precious metals of the platinum group or of Auand/or Ag to the metals less precious according to the electrochemicalseries is between 1:100 and 100:1.

The catalyst can, amongst other things, be applied to the gas diffusionlayer. Subsequently, the gas diffusion layer provided with a catalystlayer can be bonded with a polymer membrane to obtain a membraneelectrode assembly according to the invention.

In this connection, the membrane can be provided with a catalyst layeron one side or both sides. If the membrane is provided with a catalystlayer only on one side, the opposite side of the membrane has to bepressed together with an electrode which comprises a catalyst layer. Ifboth sides of the membrane are to be provided with a catalyst layer, thefollowing methods can also be applied in combination to achieve anoptimal result.

In the membrane electrode assembly according to the invention, thecatalysts contained in the electrode or the catalyst layer adjacent tothe gas diffusion layer at the side of the cathode and anode differ.

The catalyst can be applied to the membrane using customary methods,such as spraying methods and printing processes, such as for examplescreen and silk screen printing processes, inkjet printing processes,application with rollers, in particular anilox rollers, application witha slit nozzle and application with a doctor blade.

A membrane electrode assembly according to the invention exhibits asurprisingly high power density. According to a particular embodiment,preferred membrane electrode assemblies accomplish a current density ofat least 0.3 A/cm², preferably 0.4 A/cm², particularly preferably 0.5A/cm². This current density is measured in operation with pure hydrogenat the anode and air (approx. 20% by volume of oxygen, approx. 80% byvolume of nitrogen) at the cathode, with standard pressure (1013 mbarabsolute, with an open cell outlet) and a cell voltage of 0.6 V. In thisconnection, particularly high temperatures in the range of 150-200° C.,preferably 160-180° C., in particular 170° C. can be applied.

The power densities mentioned above can also be achieved with a lowstoichiometry of the fuel gases on both sides. According to a particularaspect of the present invention, the stoichiometry is less than or equalto 2, preferably less than or equal to 1.5, very particularly preferablyless than or equal to 1.2.

According to a particular embodiment of the present invention, thecatalyst layer has a low content of precious metals. The content ofprecious metals of a preferred catalyst layer which is comprised by amembrane according to the invention is preferably not more than 2mg/cm², in particular not more than 1 mg/cm², very particularlypreferably not more than 0.5 mg/cm². According to a particular aspect ofthe present invention, one side of a membrane exhibits a higher metalcontent than the opposite side of the membrane. Preference is given tothe metal content of the one side being at least twice as high as themetal content of the opposite side.

For further information on membrane electrode assemblies, reference ismade to the technical literature, in particular the U.S. Pat. No.4,191,618, U.S. Pat. No. 4,212,714 and U.S. Pat. No. 4,333,805. Thedisclosure contained in the above-mentioned citations [US-A-4,191,618,U.S. Pat. No. 4,212,714 und U.S. Pat. No. 4,333,805] with respect to thestructure and production of membrane electrode assemblies as well as theelectrodes, gas diffusion layers and catalysts to be chosen is also partof the description.

EXAMPLES Example 1

The PBI membrane was produced as described in DE 10117687 A1. Fourmembrane pieces with a size of 10 cm² were cut from the membrane. Inthis connection, a standard electrode from ETEK with a Pt loading of 1mg/cm² was used as the anode and cathode. The membrane electrodeassemblies were pressed with 1 N/mm² at 140° C. for 30 seconds.

The reference sample was installed in the cell in an untreated state.

Sample 2 was conditioned in an oven for 0.5 h at 250° C. under airatmosphere.

Sample 3 was conditioned in an oven for 1 h at 250° C. under airatmosphere.

Sample 4 was conditioned in an oven for 2 h at 250° C. under airatmosphere.

The membrane electrode assemblies were driven in at 0.3 A/cm², E-Icurves were recorded at 0 and 2 atm after one day. Table 1 compares theopen circuit voltage (OCV) and the voltages at 0.3 and 0.6 A/cm²,respectively.

TABLE 1 OCV @ mV 0 bar mV @ 0.3 A/cm² OCV @ 2 bar @ 0.6 A/cm² Reference871 520 927 624 Sample 1 935 592 961 669 Sample 2 907 575 956 652 Sample3 989 591 1014 669

Example 2

The PM membrane was produced as described in DE 10117687 A1 and amembrane electrode assembly (MEA) was prepared—as described in example1.

The MEAs are subsequently placed in an oven at a temperature of 160° C.and removed from the oven after different treatment times. The MEAs arethen titrated to determine the acid concentration. In doing so, a sampleis punched out of an MEA with a punching iron with a diameter of 2.5 cmand the electrodes are subsequently delaminated from the membrane. Thedelaminated polymer matrix is extracted in 50 ml of distilled water and30 ml of acetone at 80° C. for 30 minutes. After cooling to roomtemperature, the solution and the sample are jointly titrated with 0.1 MNaOH.

The results of an experiment are shown in table 2. In this experiment,two samples each were taken from two MEAs each and the concentration ofthe phosphoric acid was determined. Therewith, it shows that thephosphoric acid concentration can, be set via the tempering time andtemperature.

Water Phosphoric Acid Concentration Temperature Time Concentration inMEA in MEA [° C.] [min] [%] wt [%] wt Untreated — 0 74 ± 2 26 Treated160° C. 2 83 ± 5 17 Treated 160° C. 10 89 ± 2 11 Treated 160° C. 30 89 ±1 11 Treated 160° C. 900 89 ± 2 11 Treated 140° C. 10 84 ± 3 16

Example 3

In this experiment, the electrochemical output of conditioned MEAs isexamined. The measurements take place in individual cells of 50 cm².Hydrogen with a stoichiometry of 1.2 is used on the anode side, air witha stoichiometry of 2 is used on the cathode side. The measurements takeplace in ambient pressure. Current voltage characteristics of theindividual cells are recorded after an operating time of 100 h. Threeindividual cells were examined, which were equipped with an untreatedMEA, an MEA tempered for 30 min at 120° C. and an MEA tempered for 30min at 160° C., respectively.

FIG. 1 shows the current voltage characteristics. It becomes clear thatdifferent tempering conditions influence the MEA outputs in a positiveway. Thus, the measured cell voltage at 0.5 A/cm² after a tempering stepat 160° C. and 30 min is higher by 12 mV than in an untreated MEA. Aftera tempering step at 120° C. and 20 min, the cell voltage at 0.5 A/cm² isby 5 mV higher in comparison with an untreated MEA.

1-19. (canceled)
 20. A membrane electrode assembly comprising A) atleast one polymer electrolyte matrix in the form of a membranecontaining at least one oxo acid of phosphorus and/or sulphur and atleast one polymer with at least one heteroatom selected from the groupof nitrogen, oxygen and/or sulphur; and B) at least two electrodes;wherein the water content of the polymer electrolyte matrix is 20% byweight or less.
 21. The membrane electrode assembly of claim 20, whereinthe polymer electrolyte matrix is an alkaline polymer and in turncontains at least one polymer with at least one heteroatom selected fromthe group of nitrogen, oxygen and/or sulphur.
 22. The electrode of claim21, wherein the alkaline polymer is a polymer comprising at least onenitrogen atom.
 23. The membrane electrode assembly of claim 21, whereinthe alkaline polymer is a polyphosphazene, polyimine, polyisocyanide,polyetherimine, polyamide, polyhydrazide, polyurethane, polyimide,polyazole or polyazine, or a mixture thereof.
 24. The membrane electrodeassembly of claim 21, wherein the alkaline polymer is a polymer based onpolyazole, which contains the recurring azole units of the generalformula (I) and/or (II) and/or (III) and/or (IV) and/or (V) and/or (VI)and/or (VII) and/or (VIII) and/or (IX) and/or (X) and/or (XI) and/or(XII) and/or (XIII) and/or (XIV) and/or (XV) and/or (XVI) and/or (XVII)and/or (XVIII) and/or (XIX) and/or (XX) and/or (XXI) and/or (XXII)

wherein Ar are the same or different and are each a tetravalent aromaticor heteroaromatic group which may be mononuclear or polynuclear, Ar¹ arethe same or different and are each a divalent aromatic or heteroaromaticgroup which may be mononuclear or polynuclear, Ar² are the same ordifferent and are each a divalent or trivalent aromatic orheteroaromatic group which may be mononuclear or polynuclear, Ar³ arethe same or different and are each a trivalent aromatic orheteroaromatic group which may be mononuclear or polynuclear, Ar⁴ arethe same or different and are each a trivalent aromatic orheteroaromatic group which may be mononuclear or polynuclear, Ar⁵ arethe same or different and are each a tetravalent aromatic orheteroaromatic group which may be mononuclear or polynuclear, Ar⁶ arethe same or different and are each a divalent aromatic or heteroaromaticgroup which may be mononuclear or polynuclear, Ar⁷ are the same ordifferent and are each a divalent aromatic or heteroaromatic group whichmay be mononuclear or polynuclear, Ar⁸ are the same or different and areeach a trivalent aromatic or heteroaromatic group which may bemononuclear or polynuclear, Ar⁹ are the same or different and are each adivalent or trivalent or tetravalent aromatic or heteroaromatic groupwhich may be mononuclear or polynuclear, Ar¹⁰ are the same or differentand are each a divalent or trivalent aromatic or heteroaromatic groupwhich may be mononuclear or polynuclear, Ar¹¹ are the same or differentand are each a divalent aromatic or heteroaromatic group which may bemononuclear or polynuclear, X are the same or different and are eachoxygen, sulphur or an amino group which bears a hydrogen atom, a grouphaving 1-20 carbon atoms, R are identical or different and representhydrogen, an alkyl group and an aromatic group, with the proviso that Rin the formula (XX) is not hydrogen, and n and m are identical ordifferent and are each an integer greater than or equal to
 10. 25. Themembrane electrode assembly of claim 24, wherein X are the same ordifferent and are each oxygen, sulphur or an amino group which bears ahydrogen atom, a group having 1-20 carbon atoms, wherein the grouphaving 1-20 carbon atoms is a branched or unbranched alkyl or alkoxygroup, or an aryl group as further radical.
 26. The membrane electrodeassembly of claim 24, wherein n and m are identical or different and areeach an integer or equal to 100
 27. The membrane electrode assembly ofclaim 24, wherein the polymer is a copolymer or a blend which containsat least two units of the formulae (I) to (XXII) which differ from oneanother.
 28. The membrane electrode assembly of claim 24, wherein thepolymer is in the form of block copolymers, random copolymers, periodiccopolymers and/or alternating polymers.
 29. The membrane electrodeassembly of claim 20, wherein the polymer electrolyte matrix furthercomprises a further polymer.
 30. The membrane electrode assembly ofclaim 29, wherein said further polymer is at least one of the followingpolymers selected from the group consisting of polysulphone,polyethersulphone, polyaryl ketone, polyether ketone, polyether ketoneketone, polyether ether ketone, polyether ether ketone ketone and anaromatic polymer carrying sulphonic acid groups.
 31. A fuel cell systemcomprising the membrane electrode assembly of claim
 20. 32. A fuel cellstack comprising the membrane electrode assembly of claim
 20. 33. A fuelcell comprising the membrane electrode assembly of claim 20.